Heat pipes are used for cooling and transferring heat away from hot objects to a physically separated cooler area or cooling device. Heat pipes may take the form of a single closed tube, or of a pair of tubes or pipes connected in a closed circuit. The full heat pipe system also includes a volatile “working fluid” enclosed in the heat pipe, the evaporation and condensation of which mediates heat transfer from one end of the heat pipe to the other. One end of the tube or closed piping circuit is provided with a means for collecting and absorbing heat (heat absorber) at an elevated temperature, causing the fluid to evaporate. The other end of the tube or piping circuit is provided with a means for rejecting heat (heat sink) at a lower temperature than the hot end. At the heat absorber, the volatile fluid evaporates and absorbs heat; the vapor expands in volume and travels to the heat sink, where it condenses and gives up its heat. The condensed fluid returns to the hot end by gravity or capillary action via a wick, repeating the cycle. The pressure inside the heat pipe rises to that corresponding to the vapor pressure of the working fluid at the fluid boiling temperature inside the heat absorber block.
In a single tube heat pipe, the vapor travels up the core of the tube, and the liquid returns counter-currently along the walls. In a two-pipe configuration, sometimes referred to a thermosyphon, the vapor travels up one pipe, and the liquid returns via the second pipe, which is usually smaller in diameter. Heat pipes have the advantage of very high heat transfer rates, and do not rely on any mechanically moving parts.
An essential consideration in designing heat pipes is the selection of the heat exchangers for the heat absorber and the heat sink. Their characteristics determine the rate at which heat can be transferred away from hot objects or fluids. The rate of heat transfer into the heat absorber, and/or the rate of heat transfer from the heat sink to the cooling medium, is often the limiting factor in the performance of heat pipes, especially if the ratio of the fluid internal surface area to the working fluid volume is relatively small in the absorber or the heat sink
Primary considerations in selecting a heat absorber or heat sink configuration are the mechanical design of the heat transfer surfaces (e.g., configuration, material layout and thickness, orientation, etc.), the modes of heat transfer (e.g., conduction and/or convection and/or radiation), and the physical properties (especially the thermal conductivity) of the materials of construction of the heat transfer materials in direct with the working fluid.
Heat pipes have hitherto typically used simple, conventional heat exchange designs for the heat absorber and the heat sink. These include hollow blocks or plates (with a cavity for the working fluid), shell-and-tube exchangers, plate heat exchangers, bare tubes or pipes, and tubes, pipes, or hollow blocks with extended surfaces. However, despite the variety of available configurations, there has still been a need for heat pipes with higher heat transfer rates than those traditionally available.